1. Field of the Invention
This invention relates to a thin film transistor (TFT) having a planarizating film and to a liquid crystal display (LCD) having such a TFT as a switching element.
2. Description of the Prior Art
TFTs having a poly silicon film as an active layer have been developed as a driver element or a pixel driving element for use in active-matrix LCDs. An LCD having a conventional TFT will be described below.
FIG. 1 is a plan view of a conventional TFT used in a pixel section; FIG. 2 is a cross-sectional view along Cxe2x80x94C line in FIG. 1 of an LCD using such a TFT. As shown in FIG. 1, the TFT in the pixel section is formed near the intersection of gate signal line G for providing gate signals and drain signal line D for providing image signals and its source is connected to a display electrode.
The structure of such a TFT will be described with reference to FIG. 2.
A gate electrode 2 made of a refractory metal (a high melting point metal), such as chromium (Cr) and molybdenum (Mo), a gate insulating film 3, and an active layer 4 made of a poly silicon film are formed in that order on an insulating substrate 1 made of quartz glass, non-alkali glass, or the like. In the active layer 4, a channel 7 is formed over the gate electrode 2 and a source 5 and a drain 6 are formed with the channel 7 between by ion implantation with a stopper 8 on the channel 7 as a mask.
An interlayer insulating film 12 consisting of an SiO2 film 9, an SiN film 10, and an SiO2 film 11 layered in that order is formed entirely over the complete gate insulating film 3, active layer 4, and stopper 8. Furthermore, a drain electrode 33 is formed by filling a contact hole made in the interlayer insulating film 12, the position of which corresponds to the drain 6, with a metal, such as Al. In addition, a planarizating film 15 made of, for example, an organic resin for planarizating the surface is formed entirely over it. A contact hole, the position of which corresponds to the source 5, is made in the planarizating film 15. Furthermore, a transparent electrode 36, which also serves as a source electrode 34, made of ITO (indium tin oxide) being in contact with the source 5 via this contact hole is formed on the planarizating film 15.
The insulating substrate 1 on which the TFT so formed and an opposite substrate 17 having electrodes, etc. (not shown) opposite to the insulating substrate 1 are bonded together at their edges with a sealing adhesive 18 and a vacant space formed is filled with liquid crystal 19.
In the structure of such a conventional TFT, however, impurities or impurity ions generated when the sealing adhesive hardens, water or impurity ions included in the liquid crystal 19 being in contact with the planarizating film 15, water permeating from the outside through a gap 20 occurring in the sealed portion, water included in the air coming in contact with the planarizating film 15, or other substance attaches to the surface of the planarizating film 15, so that there occurs electric charge on the surface of the planarizating film 15 and polarization between the top and the bottom of the planarizating film 15 or each film of the interlayer insulating film 12 occurs.
Therefore, a back channel is formed in a TFT, resulting in the disadvantage of a change in a threshold voltage of the TFT. Furthermore, if such a TFT is used in an LCD, an increase in a threshold voltage of the TFT leads to a decrease in an ON-state current of the TFT and, for example, in a normally-white mode LCD a bright point defect may occur in which certain pixels remain always bright. Conversely, a decrease in a threshold voltage of the TFT leads to an increase in an OFF-state current of the TFT and possible dark point defects where pixels are insufficiently bright. As a result, image display is of poor quality. In addition, if a threshold voltage varies from TFT to TFT, there exists the disadvantage of being unable to obtain display having uniform brightness all over.
The present invention was made to overcome the above conventional disadvantages. That is, an object of the present invention is to provide an LCD which can display images with uniform brightness all over, by preventing polarization in a planarizating film or an interlayer insulating film of a TFT, stabilizing a threshold voltage of the TFT, and improving the bright point defect and others.
A TFT according to the present invention is a thin film transistor in which a gate electrode, a gate insulating film, a poly silicon film having a channel, an interlayer insulating film, a source electrode and a drain electrode, a planarizating film, and a transparent electrode are formed on an insulating substrate in that order. Either the source electrode or the drain electrode is formed on the interlayer insulating film so that it spreads over the channel, and/or the transparent electrode is formed on the planarizating film so that it spreads over the channel.
A TFT according to another aspect of the present invention is a thin film transistor in which a gate electrode, a gate insulating film, a poly silicon film having a channel, an interlayer insulating film, and a planarizating film are formed on a substrate in that order, a conductive film is formed on at least one of the interlayer insulating film and the planarizating film so that the conductive film covers an area over the channel, and potential of the conductive film is controlled to a predetermined value.
An LCD according to still another aspect of the present invention is one for displaying a desired image by driving liquid crystal sealed in between a pair of substrates and a substrate on which a thin film transistor is formed in the above way is used as one of the pair of substrates in this LCD. In addition, the upper surface of the planarizating film is placed opposite to the other of the pair of substrates with the liquid crystal between.
When a conductive film, such as a transparent electrode, is formed on the planarizating film so that the conductive film covers an area in the planarizating film over the channel, impurities will be prevented from attaching to an area of the planarizating film corresponding to the channel. When an area in the interlayer insulating film over the channel is covered with a conductive film, such as a source electrode or a drain electrode, polarization between the top and the bottom of the interlayer insulating film can be prevented. In addition, when the area of the planarizating film and/or the interlayer insulating film corresponding to the channel is covered with a conductive film, even if polarization occurs in the planarizating film and the potential is produced in the back channel portion of the thin film transistor, the potential of the back channel is shielded with the potential of the conductive film. Therefore, not only can the occurrence of a back channel be restrained, but, even if it occurs, variation in a threshold voltage of a TFT caused by it can be reduced. As a result, if such a TFT is used in, for example, an LCD, the bright point defect and other display defects caused by a back channel can be improved and the LCD can perform display having uniform brightness on its display surface.